Diffractive optical elements with asymmetric profiles

ABSTRACT

In an optical display system that includes a waveguide with multiple diffractive optical elements (DOEs), gratings in one or more of the DOEs may have an asymmetric profile in which gratings may be slanted or blazed. Asymmetric gratings in a DOE can provide increased display uniformity in the optical display system by reducing the “banding” resulting from optical interference that is manifested as dark stripes in the display. Banding may be more pronounced when polymeric materials are used in volume production of the DOEs to minimize system weight, but which have less optimal optical properties compared with other materials such as glass. The asymmetric gratings can further enable the optical system to be more tolerant to variations—such as variations in thickness, surface roughness, and grating geometry—that may not be readily controlled during manufacturing particularly since such variations are in the submicron range.

BACKGROUND

Diffractive optical elements (DOEs) are optical elements with a periodicstructure that are commonly utilized in applications ranging frombio-technology, material processing, sensing, and testing to technicaloptics and optical metrology. By incorporating DOEs in an optical fieldof a laser or emissive display, for example, the light's “shape” can becontrolled and changed flexibly according to application needs.

SUMMARY

In an optical display system that includes a waveguide with multiplediffractive optical elements (DOEs), gratings in one or more of the DOEsmay have an asymmetric profile in which gratings are slanted (i.e.,walls of the grating are non-orthogonal to the plane of the waveguide)or blazed. Asymmetric gratings in a DOE can provide increased displayuniformity in the optical display system by reducing the “banding”resulting from optical interference that is manifested as dark stripesin the display. Banding may be more pronounced when polymeric materialsare used in volume production of the DOEs to minimize system weight, butwhich have less optimal optical properties compared with other materialssuch as glass. Asymmetric gratings can further enable the optical systemto be more tolerant to variations—such as variations in thickness,surface roughness, and grating geometry—that may not be readilycontrolled during manufacturing, particularly since such variations arein the submicron range.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. Furthermore, the claimed subject matter is not limited toimplementations that solve any or all disadvantages noted in any part ofthis disclosure.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an illustrative near eye display systemwhich may incorporate the diffractive optical elements (DOEs) withasymmetric features;

FIG. 2 shows propagation of light in a waveguide by total internalreflection;

FIG. 3 shows a view of an illustrative exit pupil expander;

FIG. 4 shows a view of the illustrative exit pupil expander in which theexit pupil is expanded along two directions;

FIG. 5 shows an illustrative arrangement of three DOEs;

FIG. 6 shows a profile of a portion of an illustrative diffractiongrating that has straight gratings;

FIG. 7 shows an asymmetric profile of a portion of an illustrativediffraction grating that has slanted gratings;

FIGS. 8 and 9 show an illustrative arrangement for DOE fabrication;

FIGS. 10-12 show various illustrative asymmetric profiles for slantedgratings;

FIG. 13 shows an illustrative method;

FIG. 14 is a pictorial view of an illustrative example of a virtualreality or mixed reality head mounted display (HMD) device;

FIG. 15 shows a block diagram of an illustrative example of a virtualreality or mixed reality HMD device; and

FIG. 16 shows a block diagram of an illustrative electronic device thatincorporates an exit pupil expander.

Like reference numerals indicate like elements in the drawings. Elementsare not drawn to scale unless otherwise indicated.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of an illustrative near eye display system100 which may incorporate diffractive optical elements (DOEs) withasymmetric profiles. Near eye display systems are frequently used, forexample, in head mounted display (HMD) devices in industrial,commercial, and consumer applications. Other devices and systems mayalso use DOEs with asymmetric profiles, as described below, and it isemphasized that the near eye display system 100 is intended to be anexample that is used to illustrate various features and aspects, and thepresent DOEs are not necessarily limited to near eye display systems.

System 100 may include an imager 105 that works with an optical system110 to deliver images as a virtual display to a user's eye 115. Theimager 105 may include, for example, RGB (red, green, blue) lightemitting diodes (LEDs), LCOS (liquid crystal on silicon) devices, OLED(organic light emitting diode) arrays, MEMS (micro-electro mechanicalsystem) devices, or any other suitable displays or micro-displaysoperating in transmission, reflection, or emission. The imager 105 mayalso include mirrors and other components that enable a virtual displayto be composed and provide one or more input optical beams to theoptical system. The optical system 110 can typically include magnifyingoptics 120, pupil forming optics 125, and one or more waveguides 130.

In a near eye display system the imager does not actually shine theimages on a surface such as glass lenses to create the visual displayfor the user. This is not feasible because the human eye cannot focus onsomething that is that close. Indeed, rather than create a visible imageon a surface, the near eye optical system 100 uses the pupil formingoptics 125 to form a pupil and the eye 115 acts as the last element inthe optical chain and converts the light from the pupil into an image onthe eye's retina as a virtual display.

The waveguide 130 facilitates light transmission between the imager andthe eye. One or more waveguides can be utilized in the near eye displaysystem because they are transparent and because they are generally smalland lightweight (which is desirable in applications such as HMD deviceswhere size and weight is generally sought to be minimized for reasons ofperformance and user comfort). For example, the waveguide 130 can enablethe imager 105 to be located out of the way, for example, on the side ofthe head, leaving only a relatively small, light, and transparentwaveguide optical element in front of the eyes. In one implementation,the waveguide 130 operates using a principle of total internalreflection, as shown in FIG. 2, so that light can be coupled among thevarious optical elements in the system 100.

FIG. 3 shows a view of an illustrative exit pupil expander (EPE) 305.EPE 305 receives an input optical beam from the imager 105 throughmagnifying optics 120 to produce one or more output optical beams withexpanded exit pupil in one or two dimensions relative to the exit pupilof the imager (in general, the input may include more than one opticalbeam which may be produced by separate sources). The expanded exit pupiltypically facilitates a virtual display to be sufficiently sized to meetthe various design requirements of a given optical system, such as imageresolution, field of view, and the like, while enabling the imager andassociated components to be relatively light and compact.

The EPE 305 is configured, in this illustrative example, to supportbinocular operation for both the left and right eyes (components whichmay be utilized for stereoscopic operation such as scanning mirrors,lenses, filters, beam splitters, MEMS devices, or the like are not shownin FIG. 3 for sake of clarity in exposition). Accordingly, the EPE 305utilizes two out-coupling gratings, 310 _(L) and 310 _(R) that aresupported on a waveguide 330 and a central in-coupling grating 340. Thein-coupling and out-coupling gratings may be configured using multipleDOEs, as described in the illustrative example below. While the EPE 305is depicted as having a planar configuration, other shapes may also beutilized including, for example, curved or partially spherical shapes,in which case the gratings disposed thereon are non-co-planar.

As shown in FIG. 4, the EPE 305 may be configured to provide an expandedexit pupil in two directions (i.e., along each of a first and secondcoordinate axis). As shown, the exit pupil is expanded in both thevertical and horizontal directions. It may be understood that the terms“direction,” “horizontal,” and “vertical” are used primarily toestablish relative orientations in the illustrative examples shown anddescribed herein for ease of description. These terms may be intuitivefor a usage scenario in which the user of the near eye display device isupright and forward facing, but less intuitive for other usagescenarios. Accordingly, the listed terms are not to be construed tolimit the scope of the configurations (and usage scenarios therein) ofDOEs with asymmetric grating features.

FIG. 5 shows an illustrative arrangement of three DOEs that may be usedas part of a waveguide to provide in-coupling and expansion of the exitpupil in two directions. Each DOE is an optical element comprising aperiodic structure that can modulate various properties of light in aperiodic pattern such as the direction of optical axis, optical pathlength, and the like. The first DOE, DOE 1 (indicated by referencenumeral 505), is configured to couple the beam from the imager into thewaveguide. The second DOE, DOE 2 (510), expands the exit pupil in afirst direction along a first coordinate axis, and the third DOE, DOE 3(515), expands the exit pupil in a second direction along a secondcoordinate axis and couples light out of the waveguide. The angle ρ is arotation angle between the periodic lines of DOE 2 and DOE 3 as shown.DOE 1 thus functions as an in-coupling grating and DOE 3 functions as anout-coupling grating while expanding the pupil in one direction. DOE 2may be viewed as an intermediate grating that functions to couple lightbetween the in-coupling and out-coupling gratings while performing exitpupil expansion in the other direction. Using such intermediate gratingmay eliminate a need for conventional functionalities for exit pupilexpansion in an EPE such as collimating lenses.

Some near eye display system applications, such as those using HMDdevices for example, can benefit by minimization of weight and bulk. Asa result, the DOEs and waveguides used in an EPE may be fabricated usinglightweight polymers. Such polymeric components can support design goalsfor size, weight, and cost, and generally facilitate manufacturability,particularly in volume production settings. However, polymeric opticalelements generally have lower optical resolution relative to heavierhigh quality glass. Such reduced optical resolution and the waveguide'sconfiguration to be relatively thin for weight savings and packagingconstraints within a device can result in optical interference whichappears as a phenomena referred to as “banding” in the display. Theoptical interference that results in banding arises from lightpropagating within the EPE that has several paths to the same location,in which the optical path lengths differ.

The banding is generally visible in the form of dark stripes whichdecrease optical uniformity of the display. Their location on thedisplay may depend on small nanometer-scale variations in the opticalelements including the DOEs in one or more of thickness, surfaceroughness, or grating geometry including grating line width, angle, fillfactor, or the like. Such variation can be difficult to characterize andmanage using tools that are generally available in manufacturingenvironments, and particularly for volume production. Conventionalsolutions to reduce banding include using thicker waveguides which canadd weight and complicate package design for devices and systems. Othersolutions use pupil expansion in the EPE in just one direction which canresult in a narrow viewing angle and heightened sensitivity to naturaleye variations among users.

By comparison, when one or more of the DOEs 505, 510, and 515 (FIG. 5)are configured with gratings that have an asymmetric profile, bandingcan be reduced even when the DOEs are fabricated from polymers and thewaveguide 130 (FIG. 1) is relatively thin. FIG. 6 shows a profile ofstraight (i.e., non-slanted) grating features 600 (referred to asgrating bars, grating lines, or simply “gratings”), that are formed in asubstrate 605. By comparison, FIG. 7 shows grating features 700 formedin a substrate 705 that have an asymmetric profile. That is, thegratings may be slanted (i.e., non-orthogonal) relative to a plane ofthe waveguide. In implementations where the waveguide is non-planar,then the gratings may be slanted relative to a direction of lightpropagation in the waveguide. Asymmetric grating profiles can also beimplemented using blazed gratings, or echelette gratings, in whichgrooves are formed to create grating features with asymmetric triangularor sawtooth profiles.

In FIGS. 6 and 7, the grating period is represented by d, the gratingheight by h, bar width by c, and the filling factor by f, where f=c/d.The slanted gratings in FIG. 7 may be described by slant angles α₁ andα₂. In one exemplary embodiment, for a DOE, d=390 nm, c=d/2, h=300 nm,α₁=α₂=45 degrees, f=0.5, and the refractive index of the substratematerial is approximately 1.71. In other implementations, ranges ofsuitable values may include d=250 nm-450 nm, h=200 nm-400 nm, f=0.3-0.0,and α₁=30-50 degrees, with refractive indices of 1.7 to 1.9. In anotherexemplary embodiment, DOE 2 is configured with portions that haveasymmetric profiles, while DOE 1 and DOE 3 are configured withconventional symmetric profiles using straight gratings.

By slanting the gratings in one or more of the DOEs 505, 510, and 515,banding can be reduced to increase optical uniformity while enablingmanufacturing tolerances for the DOEs to be less strict, as comparedwith using the straight grating features shown in FIG. 6 for the samelevel of uniformity. That is, the slanted gratings shown in FIG. 7 aremore tolerant to manufacturing variations noted above than the straightgratings shown in FIG. 6, for comparable levels of optical performance(e.g., optical resolution and optical uniformity).

FIGS. 8 and 9 show an illustrative arrangement for DOE fabrication usinga substrate holder 805 that rotates a grating substrate 810 about anaxis 815 relative to a reactive ion etching plasma source 820. Exposureto the plasma may be used, for example, to adjust the thickness andorientation of the etching on the grating substrate at various positionsby angling the substrate relative to the source as shown in FIG. 9using, for example, a computer-controller or other suitable controlsystem (not shown). In an illustrative example, the etching may beperformed using a reactive ion beam etching (RIBE). However, othervariations of ion beam etching may be utilized in variousimplementations including, for example, magnetron reactive ion etching(MRIE), high density plasma etching (HDP), transformer coupled plasmaetching (TCP), inductively coupled plasma etching (ICP), and electroncyclotron resonance plasma etching (ECR).

By controlling the exposure of the substrate to the plasma, gratingangle and depth can be controlled to create a slanted microstructure onthe substrate. The microstructure may be replicated for mass productionin a lightweight polymer material using one of cast-and-cure, embossing,compression molding, or compression injection molding, for example.

Ion beam etching may produce variations from the idealized grating shownin FIG. 6 in which the gratings have parallel walls. The profile 1000 inFIG. 10 includes non-parallel sidewalls (representatively indicated byreference numeral 1005) that are undercut and the profile 1100 in FIG.11 includes non-parallel sidewalls 1105 that are overcut. The change inangle of the sidewalls is denoted by β, as shown in FIG. 10, and apositive value of β implies undercutting while a negative value of βimplies overcutting. Compensation for the effects of undercutting andovercutting can be realized in some implementations by ensuring that afill factor f_(mid) in the center of the feature meets the design valuefor the grating, as shown in profile 1200 in FIG. 12. Here, the gratingwalls essentially pivot about this center position as β varies.

FIG. 13 is a flowchart 13 of an illustrative method 1300. Unlessspecifically stated, the methods or steps shown in the flowchart anddescribed in the accompanying text are not constrained to a particularorder or sequence. In addition, some of the methods or steps thereof canoccur or be performed concurrently and not all the methods or steps haveto be performed in a given implementation depending on the requirementsof such implementation and some methods or steps may be optionallyutilized.

In step 1305, light is received at an in-coupling DOE. The in-couplinggrating is disposed in an EPE and interfaces with the downstreamintermediate DOE that is disposed in the EPE. In step 1310, the exitpupil of the received light is expanded along a first coordinate axis inthe intermediate DOE. The intermediate DOE is configured with gratingshaving an asymmetric profile such as slanted gratings or blazedgratings. In step 1315, the exit pupil is expanded in an out-couplingDOE which outputs light with an expanded exit pupil relative to thereceived light at the in-coupling DOE along the first and secondcoordinate axes in step 1320. The intermediate DOE is configured tointerface with a downstream out-coupling DOE. In some implementations,the out-coupling DOE may be apodized with shallow gratings that areconfigured to be either straight or slanted.

DOEs with asymmetric profiles may be incorporated into a display systemthat is utilized in a virtual or mixed reality display device. Suchdevice may take any suitable form, including but not limited to near-eyedevices such as an HMD device. A see-through display may be used in someimplementations while an opaque (i.e., non-see-through) display using acamera-based pass-through or outward facing sensor, for example, may beused in other implementations.

FIG. 14 shows one particular illustrative example of a see-through,mixed reality or virtual reality display system 1400, and FIG. 15 showsa functional block diagram of the system 1400. Display system 1400comprises one or more lenses 1402 that form a part of a see-throughdisplay subsystem 1404, such that images may be displayed using lenses1402 (e.g. using projection onto lenses 1402, one or more waveguidesystems incorporated into the lenses 1402, and/or in any other suitablemanner). Display system 1400 further comprises one or moreoutward-facing image sensors 1406 configured to acquire images of abackground scene and/or physical environment being viewed by a user, andmay include one or more microphones 1408 configured to detect sounds,such as voice commands from a user. Outward-facing image sensors 1406may include one or more depth sensors and/or one or more two-dimensionalimage sensors. In alternative arrangements, as noted above, a mixedreality or virtual reality display system, instead of incorporating asee-through display subsystem, may display mixed reality or virtualreality images through a viewfinder mode for an outward-facing imagesensor.

The display system 1400 may further include a gaze detection subsystem1410 configured for detecting a direction of gaze of each eye of a useror a direction or location of focus, as described above. Gaze detectionsubsystem 1410 may be configured to determine gaze directions of each ofa user's eyes in any suitable manner. For example, in the illustrativeexample shown, a gaze detection subsystem 1410 includes one or moreglint sources 1412, such as infrared light sources, that are configuredto cause a glint of light to reflect from each eyeball of a user, andone or more image sensors 1414, such as inward-facing sensors, that areconfigured to capture an image of each eyeball of the user. Changes inthe glints from the user's eyeballs and/or a location of a user's pupil,as determined from image data gathered using the image sensor(s) 1414,may be used to determine a direction of gaze.

In addition, a location at which gaze lines projected from the user'seyes intersect the external display may be used to determine an objectat which the user is gazing (e.g. a displayed virtual object and/or realbackground object). Gaze detection subsystem 1410 may have any suitablenumber and arrangement of light sources and image sensors. In someimplementations, the gaze detection subsystem 1410 may be omitted.

The display system 1400 may also include additional sensors. Forexample, display system 1400 may comprise a global positioning system(GPS) subsystem 1416 to allow a location of the display system 1400 tobe determined. This may help to identify real world objects, such asbuildings, etc. that may be located in the user's adjoining physicalenvironment.

The display system 1400 may further include one or more motion sensors1418 (e.g., inertial, multi-axis gyroscopic, or acceleration sensors) todetect movement and position/orientation/pose of a user's head when theuser is wearing the system as part of a mixed reality or virtual realityHMD device. Motion data may be used, potentially along with eye-trackingglint data and outward-facing image data, for gaze detection, as well asfor image stabilization to help correct for blur in images from theoutward-facing image sensor(s) 1406. The use of motion data may allowchanges in gaze location to be tracked even if image data fromoutward-facing image sensor(s) 1406 cannot be resolved.

In addition, motion sensors 1418, as well as microphone(s) 1408 and gazedetection subsystem 1410, also may be employed as user input devices,such that a user may interact with the display system 1400 via gesturesof the eye, neck and/or head, as well as via verbal commands in somecases. It may be understood that sensors illustrated in FIGS. 14 and 15and described in the accompanying text are included for the purpose ofexample and are not intended to be limiting in any manner, as any othersuitable sensors and/or combination of sensors may be utilized to meetthe needs of a particular implementation. For example, biometric sensors(e.g., for detecting heart and respiration rates, blood pressure, brainactivity, body temperature, etc.) or environmental sensors (e.g., fordetecting temperature, humidity, elevation, UV (ultraviolet) lightlevels, etc.) may be utilized in some implementations.

The display system 1400 can further include a controller 1420 having alogic subsystem 1422 and a data storage subsystem 1424 in communicationwith the sensors, gaze detection subsystem 1410, display subsystem 1404,and/or other components through a communications subsystem 1426. Thecommunications subsystem 1426 can also facilitate the display systembeing operated in conjunction with remotely located resources, such asprocessing, storage, power, data, and services. That is, in someimplementations, an HMD device can be operated as part of a system thatcan distribute resources and capabilities among different components andsubsystems.

The storage subsystem 1424 may include instructions stored thereon thatare executable by logic subsystem 1422, for example, to receive andinterpret inputs from the sensors, to identify location and movements ofa user, to identify real objects using surface reconstruction and othertechniques, and dim/fade the display based on distance to objects so asto enable the objects to be seen by the user, among other tasks.

The display system 1400 is configured with one or more audio transducers1428 (e.g., speakers, earphones, etc.) so that audio can be utilized aspart of a mixed reality or virtual reality experience. A powermanagement subsystem 1430 may include one or more batteries 1432 and/orprotection circuit modules (PCMs) and an associated charger interface1434 and/or remote power interface for supplying power to components inthe display system 1400.

It may be appreciated that the display system 1400 is described for thepurpose of example, and thus is not meant to be limiting. It is to befurther understood that the display device may include additional and/oralternative sensors, cameras, microphones, input devices, outputdevices, etc. than those shown without departing from the scope of thepresent arrangement. Additionally, the physical configuration of adisplay device and its various sensors and subcomponents may take avariety of different forms without departing from the scope of thepresent arrangement.

As shown in FIG. 16, an EPE incorporating DOEs with asymmetric profilescan be used in a mobile or portable electronic device 1600, such as amobile phone, smartphone, personal digital assistant (PDA),communicator, portable Internet appliance, hand-held computer, digitalvideo or still camera, wearable computer, computer game device,specialized bring-to-the-eye product for viewing, or other portableelectronic device. As shown, the portable device 1600 includes a housing1605 to house a communication module 1610 for receiving and transmittinginformation from and to an external device, or a remote system orservice (not shown).

The portable device 1600 may also include an image processing module1615 for handling the received and transmitted information, and avirtual display system 1620 to support viewing of images. The virtualdisplay system 1620 can include a micro-display or an imager 1625 and anoptical engine 1630. The image processing module 1615 may be operativelyconnected to the optical engine 1630 to provide image data, such asvideo data, to the imager 1625 to display an image thereon. An EPE 1635using one or more DOEs with asymmetric profiles can be optically linkedto an optical engine 1630.

An EPE using one or more DOEs with asymmetric profiles may also beutilized in non-portable devices, such as gaming devices, multimediaconsoles, personal computers, vending machines, smart appliances,Internet-connected devices, and home appliances, such as an oven,microwave oven and other appliances, and other non-portable devices.

Various exemplary embodiments of the present diffractive opticalelements with asymmetric profiles are now presented by way ofillustration and not as an exhaustive list of all embodiments. Anexample includes an optical system, comprising: a substrate of opticalmaterial; a first diffractive optical element (DOE) disposed on thesubstrate, the first DOE having an input surface and configured as anin-coupling grating to receive one or more optical beams as an input;and a second DOE disposed on the substrate and configured for pupilexpansion of the one or more optical beams along a first direction,wherein at least a portion of the second DOE includes gratings that areconfigured with a predetermined slant angle to a direction orthogonal toa plane of the substrate.

In another example, the optical system further includes a third DOEdisposed on the substrate, the third DOE having an output surface andconfigured for pupil expansion of the one or more optical beams along asecond direction, and further configured as an out-coupling grating tocouple, as an output from the output surface, one or more optical beamswith expanded pupil relative to the input. In another example, at leasta portion of the third DOE includes gratings that are configured with asecond predetermined slant angle to a direction orthogonal to a plane ofthe output surface. In another example, at least a portion of the firstDOE includes gratings that are configured with a third predeterminedslant angle to a direction orthogonal to a plane of the input surface.In another example, the one or more optical beams received at the firstDOE emanate as a virtual image produced by a micro-display or imager.

A further example includes an electronic device, comprising: a dataprocessing unit; an optical engine operatively connected to the dataprocessing unit for receiving image data from the data processing unit;an imager operatively connected to the optical engine to form imagesbased on the image data and to generate one or more input optical beamsincorporating the images; and an exit pupil expander, responsive to theone or more input optical beams, comprising a structure on whichmultiple diffractive optical elements (DOEs) are disposed, in which theexit pupil expander is configured to provide one or more output opticalbeams, using one or more of the DOEs, as a near eye virtual display withan expanded exit pupil, and wherein at least one of the DOEs isconfigured with gratings having an asymmetric profile.

In another example, the asymmetric profile comprises one of gratingswith slanted sidewalls or blazed gratings. In another example, the exitpupil expander provides pupil expansion in two directions. In anotherexample, the imager includes one of light emitting diode, liquid crystalon silicon device, organic light emitting diode array, or micro-electromechanical system device. In another example, the imager comprises amicro-display operating in one of transmission, reflection, or emission.In another example, the electronic device is implemented in a headmounted display device or portable electronic device. In anotherexample, each of the one or more input optical beams is produced by acorresponding one or more sources. In another example, the structure iscurved or partially spherical. In another example, two or more of theDOEs are non-co-planar.

A further example includes a method, comprising: receiving light at anin-coupling diffractive optical element (DOE) disposed in an exit pupilexpander; expanding an exit pupil of the received light along a firstcoordinate axis in an intermediate DOE disposed in the exit pupilexpander; expanding the exit pupil along a second coordinate axis in anout-coupling DOE disposed in the exit pupil expander; and outputtinglight with an expanded exit pupil relative to the received light at thein-coupling DOE along the first and second coordinate axes using theout-coupling DOE, in which the intermediate DOE is configured withgratings that have non-orthogonal orientation relative to a plane of theexit pupil expander.

In another example, the non-orthogonal orientation comprises a slantangle of between 30 and 50 degrees from a normal to the plane. Inanother example, the in-coupling DOE, the intermediate DOE, or theout-coupling DOE is formed with a polymer that is molded from asubstrate that is etched using ion beam etching in which the substrateis rotatable relative to an ion beam source. In another example, atleast a portion of the out-coupling DOE is an apodized diffractiongrating having shallow grooves relative to the in-coupling DOE or theintermediate DOE. In another example, the method is performed in a neareye display system. In another example, the output light provides avirtual display to a user of the near eye display system.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed:
 1. An optical display system, comprising: a substrateof planar optical material; a first diffractive optical element (DOE)arranged on the substrate, the first DOE having an input surface andconfigured as an in-coupling grating to receive one or more opticalbeams as an input; and a second DOE arranged on the substrate whichreceives one or more optical beams from the first DOE and couples thereceived one or more optical beams to a third DOE, and in which thesecond DOE is configured as an intermediate DOE for pupil expansion ofthe received one or more optical beams along a first direction, whereinat least a portion of the second DOE includes grating features eachhaving a slant angle to a respective axis orthogonal to the plane of thesubstrate, such that each grating feature is asymmetric about the axis,and wherein the second DOE is configured with uniform grating featuresthat are periodic in only a single direction.
 2. The optical displaysystem of claim 1 in which the third DOE is arranged on the substrate,the third DOE having an output surface and configured for pupilexpansion of the one or more optical beams along a second direction, andfurther configured as an out-coupling grating to couple, as an outputfrom the output surface, one or more optical beams with expanded pupilrelative to the input.
 3. The optical display system of claim 2 in whichat least a portion of the third DOE includes gratings that areconfigured with a second slant angle to a direction orthogonal to aplane of the output surface.
 4. The optical display system of claim 3 inwhich at least a portion of the first DOE includes gratings that areconfigured with a third slant angle to a direction orthogonal to a planeof the input surface.
 5. The optical display system of claim 1 in whichthe one or more optical beams received at the first DOE emanate as avirtual image produced by a micro-display or imager.